For those who have students working Lab 1a in class together, we created this optional instruction sheet for students to get their Salesforce accounts in advance of starting Lab 1a.
When many students are trying to get their accounts at the same time, Salesforce's account-generating server can get bogged down. In our recent experience, with 40-50 students in class, it was taking 5-10 minutes for students to receive the Welcome emails. But one or two took longer–as much as 15-20 minutes in one case.
If you're working 1a in the classroom, you may want to give students this instruction sheet so they just get their account in advance to avoid that issue.
Note: When students are working the lab as homework, on their own time, the account requests are not bunched together so the wait times should be shorter.